Assemblies making use of an axially movable member threadedly mounted on a screw and connected to a spiral spring motor, in lowering and raising a window shade are illustrated by U.S. Pat. No. 4,427,050 to Toppen and U.S. Pat. No. 4,523,620 to Mortellite. As the axially movable member rotates when the shade is lowered, the spring motor is increased in tension. Also known as exemplified by U.S. Pat. No. 5,484,051 to Nagai et al, is the use of an axially movable member threadedly mounted on a rotatable ball screw shaft coupled to a rotatable shaft of an electric motor, with rodless cylinders including pistons movable by fluid pressure, for vertically moving a workpiece. In addition, as illustrated by U.S. Pat. No. 5,370,367 to Zaguroli, Jr., an axially movable ball nut threadedly mounted on a ball screw fixed within a drum reel for a cable, is known to be rotatable by regulated air pressure applied to the drum reel. In an embodiment of the zaguroli device, piston and cylinder shock absorbers pressurized by a source of compressed air, oppose cable wind up.
However, none of this prior art provides a barrier attached, in use, at both ends to support surfaces, and extendable and retractable beneficially in response to a changing environment of use. In such case, extension or retraction of the barrier as needed, could prevent damage to, or tearing or even destruction of, the barrier. Moreover, none of this prior art provides a barrier in combination with an extension and retraction mechanism that exerts substantially constant force regardless of the barrier extent. Furthermore, none of this prior art provides a barrier useful as a visual barrier and extending between coupled passenger rail cars for reducing the possibility that a visually impaired person intending to step into a rail car, might accidentally step off a train platform onto the tracks.